1. Field of the Invention
The present invention concerns a device and a process for setting the focal spot position of the electron beam on the anode of an X-ray tube.
2. Description of the Prior Art
X-ray tubes are used in X-ray devices in order to generate X-radiation. Electrons from a cathode are accelerated in the X-ray tube through an electric field at the X-ray voltage to an anode. When they strike the anode, the electrons create characteristic X-radiation as a result of their kinetic energy. The direction and shape of the X-rays that are generated are determined by the quality and orientation of the surface of the anode as well as by the direction and focal spot position of the electron beam when it strikes the anode. In order to create a bundled and intensive X-ray beam in the desired direction, the electron beam is focused and directed at a specific point on the anode surface.
While electric fields are also used to focus the electron beam, its deflection is usually caused by magnetic fields. These are created by deflection coils, which are arranged between the cathode and the anode around the electron beam or the X-ray tube. Depending on the requirements for the sharpness of the focus, the complexity of the focal spot contour, and the options for deflecting the electron beam, one or more deflection coil are provided.
The magnetic field created by the coils varies depending on the coil current. Variations of the coil current thus cause variations in the focal spot position. With application-dependent changes in the X-ray voltage, with which the electrons are accelerated from the cathode to the anode of the X-ray tube, the coil current must also be changed in order to attain the retention of the focal spot position; thus, the coil current vanes depending on the X-ray voltage.
For permanent retention of the desired focal spot position, the coil current of the X-ray voltage must be updated sufficiently quickly. It must also be supplied sufficiently exactly in order to ensure a stable focal spot position. Moreover, fluctuations in the X-ray voltage must be correctable by changing the coil current and an appropriate behavior must be ensured during failures in the X-ray voltage as a result of X-ray arcing.
Regardless of how the electric or magnetic fields for deflection of the electron beam are created, their field strength must take into consideration the currently existing X-ray voltage. The X-ray voltage can either be tapped from the voltage generator, which requires an additional connection between the generator and the deflection device, or it can be tapped from the current X-ray tube voltage. For this, a voltage divider, from which a signal can be tapped proportional to the X-ray voltage, is connected between the high voltage and ground either on the X-ray tube or on the X-ray generator on the anode side as well as on the cathode side.
A control signal, which sets the strength of the electric or magnetic fields for deflecting the electron beam, is generated from this signal from a control electronics assembly in accordance with a stored characteristic curve. If several deflection devices are available in different orientations, then the orientation of the deflection through the characteristic curves is taken into consideration. This concerns control in the classic sense, for which there is no mutual dependency between the focal spot position as the controlled variable and the deflection field as the controlled parameter.
The voltage dividers for tapping the signal proportional to the X-ray voltage represent stray capacitances, which vary over time and are susceptible to disturbances. The electrical connection between the deflection control and the tapping of the high voltage signal also possesses stray capacitances and disturbance inductivities as possible sources of errors. Last but not least, the production tolerances of the X-ray tube, fluctuations in the voltage supply of the X-ray generator, and other unforeseeable disturbances create sources of errors. None these unforeseeable disturbances are taken into consideration in the characteristic curve that is stored for the deflection control and are thus not compensated from the very outset.